Polycarbonates are well-known as excellent materials for optical applications because of their inherent toughness and clarity. The most familiar linear polycarbonates are homopolymers derived from 2,2-bis(4-hydroxyphenyl)propane, commonly known as bisphenol-A (hereinafter, BPA) and illustrated by the following structure: ##STR1##
BPA-based homopolymeric materials are transparent and exhibit excellent mechanical properties. Thus, they are often used in the fabrication of optical materials such as lenses and substrates for optical storage media.
During manufacturing, the polymeric polycarbonate is typically molded at high temperatures and pressures which, upon cooling, may lead to molecular orientations and stresses that are frozen into the material. In such cases, the cooled polycarbonate becomes anisotropic and exhibits orientational birefringence. As a light ray passes through a birefringent material, it is split into two plane-polarized light rays, each having a plane of polarization extending in a perpendicular direction relative to the other. Each light ray has a different index of refraction in the polymer, and the difference between these indices of refraction is referred to as the birefringence of the material. Because light passing through a birefringent material may follow more than one path, distortion of the light may result. Thus, birefringence is an undesirable property for polymers used in optical applications. Ideally, materials used in optical applications should have a birefringence substantially equal to zero.
Wimberger Friedl et al. reported in U.S. Pat. No. 5,424,389 and European Patent Application 0621297A2 random copolycarbonates of BPA and 6,6'-dihydroxy-3,3,3',3'-tetramethyl-1,1'-spirobiindane (hereinafter SBI). SBI is represented by the following structure: ##STR2##
Wimberger Friedl et al. found that when the mole fraction of SBI in the random SBI/BPA copolycarbonate varies from 0.844 to 0.887, there are dramatic improvements in birefringence over BPA polycarbonate homopolymers, as measured by the stress-optical coefficient (C.sub.m). The stress-optical coefficient (C.sub.m) for a polymer is a measure of its sensitivity to orientational birefringence. Preferably, the absolute value of C.sub.m in polymers used in optical applications is substantially equal to zero. Similarly, Faler et al. disclosed in U.S. Pat. No. 4,950,731 that random SBI/BPA copolymers demonstrate improved optical properties as compared with BPA polycarbonates.
In addition, high glass transition temperature values (T.sub.g) are critical for molded polycarbonate resins that undergo high temperature processing (&gt;1500.degree. C.) to maintain the integrity of the molded part. Such high temperature processing occurs, for example, during the application of chemically resistant hard coats or thick surface coatings often used in optical applications and during the vapor deposition of coatings which have specific optical device utilities.
SBI homopolycarbonates exhibit a high T.sub.g (up to 230.degree. C.), as disclosed in the aforementioned patent to Faler et al., but the mechanical strength and ductility of SBI materials are much reduced relative to the BPA polycarbonates. By contrast, commercially available polycarbonate resins based solely on BPA exhibit excellent optical and mechanical properties, but they are unsuitable for high temperature applications or further high temperature surface processing because of their relatively low glass transition temperature values of approximately 150.degree. C.
However, by adding and varying the amount of BPA monomer in spirobiindane (SBI) based polycarbonates, Faler et al. reported that the low T.sub.g of BPA polycarbonates can be counteracted. These SBI and BPA monomers used in combination produce random copolymers with a high T.sub.g between 164 and 218.degree. C. Random SBI/BPA copolymers or copolycarbonates, as used herein, refer to polymers in which SBI and BPA monomers are randomly distributed in the polymeric backbone chain. Thus, in random polymers, the arrangement of the monomers which comprise the backbone composition cannot be controlled.
An optical waveguide or optical fiber, useful in the field of optical information transmission, typically comprises an outer cladding material having a central channel that is filled with a light-transmitting core material exhibiting a higher index of refraction. To promote internal reflection, the light transmitting core material must have an index of refraction that is higher than that of the cladding material by at least 0.3%. Because of the desirable optical and thermal properties described above, copolymers of SBI and BPA are potentially useful in the fabrication of waveguides. As used herein, index of refraction and refractive index are interchangeable.
The present invention is based on the unexpected discovery of high molecular weight linear polycarbonates derived from various spirobiindane bisphenol and dihydroxyaromatic derivatives that can be used in combination with random SBI/BPA polycarbonates to create optical waveguides and optical fibers. In particular, the indices of refraction of the present polymers differ by at least 0.3% from those of known SBI/BPA random polycarbonates. Thus, the polymers of the present invention can be used as either core or cladding materials in optical fibers employing random SBI/BPA polycarbonates as respective cladding or core.
The novel polycarbonate polymers of the present invention, as shown in the following structure (I), comprise alternating blocks or units of the aforementioned derivatives. The composition of the polymeric backbone is uniform and exactly controlled. In addition, the present polycarbonates are clear and ductile exhibiting high glass transition temperatures and excellent optical properties.